1. Field of the Invention
This invention relates to limiter circuits implemented with op-amps or comparators. More particularly, it relates to such limiter circuits having negative feedback which eliminates the effects of offset and optional positive feedback which establishes a selectable threshold.
2. Background Information
Limiter circuits generate sharp rectangular waves with a fixed amplitude from a periodic signal such as a sine wave. They can also be used to sharpen the edges of square wave signals. A typical application for a limiter circuit is at the head of the frequency detection means in an FM or frequency shift keying (FSK) system to eliminate any amplitude sensitivity. A limiter circuit may be used, for example, in a data receiver to convert a variable amplitude sine wave input to a constant amplitude square wave. Conventionally, limiter circuits have utilized clipping diodes or an overdriven amplifier, and more particularly an op-amp. Such circuits are passive. Often a comparator is used as the amplifier. The comparator has a higher internal gain than a typical op-amp, and is intended to be operated rail to rail, while the op-amp typically operates in the linear region.
The op-amps and comparators typically used in limiters have a dc offset. This offset places a lower limit on the signal amplitude which may be processed. Further, when the signal amplitude is just slightly larger than the offset voltage, even harmonics will be present in the limiter output, which may degrade the operation of the subsequent frequency detection means. Thus, the offset limits the dynamic range of the limiter circuit. Higher quality op-amps have lower offset and therefore less error. However, use of a high quality op-amp significantly increases the cost of a limiter circuit and does not completely eliminate the offset. Comparators with very low offset are not available.
There is a need, therefore, for an improved limiter circuit which provides an increased dynamic range at a reasonable cost.
There are some applications in which the circuit receiving the output from the limiter can only accept an input signal at either of two levels, or where noise appearing at the limiter circuit output would present a problem.
Therefore, there is also a need for some applications for an improved limiter circuit which has a selectable threshold level.